338 APPLICATION OF PRINCIPLES. 



Manures act apparently in one of three ways, 

 either by merely stimulating the vital forces, as corn- 

 raised. Peas, and many other leguminous plants, take very little 

 alkaline or earthy matter from the soil : they contain no potash, 

 and only a trace of phosphate of lime or magnesia^ This explains 

 their utility as fallaw-crops : they return to the soil a certain portion 

 of vegetable mould, while they scarcely if at all diminish the potash 

 or tlie phosphates of the soil, wliich are required for the succeeding 

 wheat crop. " When we grow in the same poil, for several yeiirs in 

 succession, different plants, the first of which leaves jehind that 

 which the second, and the second that which the third, may require, 

 the soil will be a fruitful one for all the three kinds of produce. If 

 the first plant, for example, be Wheat, which consumes tlie greatest 

 part of the silicate of potash in a soil, while the plants which suc- 

 ceed it are of such a kind as require only small quantities of potash, 

 as is the case with the Leguminosce [Pea tribe], Turnips, Potatoes, 

 <fec., the wheat may be again sowed with advantage after tlie fourth 

 year ; for, during the interval of three years, the soil will be render- 

 ed capable of again yielding silicate of potash in sufiicient quantity." 

 (^Liebiff, Organic Chetnistrt/, p. 170.) Tlie alkaline constituents of 

 the soil, except when added in the form of manure, are derived from 

 the slow disintegration and decomposition of the rocks which com- 

 pose it, and particularly the argillaceous earths. According to 

 Liebig, the quantity of potash contained in a layer of soil formed by 

 the disintegration of 40,000 square feet of the following rocks to the 

 depth of 20 inches, is as follows ; 



Felspar contains 1,152,000 lbs. 



Clinkstone contains .... from 200,000 to 400,000 lbs. 



Basalt contains from 47,500 to 76,000 lbs. 



Clay-slate contains from 100,000 to 200,000 lbs. 



Loam contains from 87,000 to 300,000 lbs. 



" A thousandth part of loam, mixed with the quartz in new red 

 sandstone, or with the lime in tlie different limestone formations, 

 affords as much potash to a soil only 20 inches in depth as is sufficient 

 to supply a forest of pines growing upon it for a century. A single 

 cubic foot of felspar is sufficient to supply a wood, coveripg a surface 

 of 400,000 square feet, with the potash required for five years." 



